The invention relates to a process to convert copper or nickel mattes into substantially pure metal by blowing oxygen-enriched air into a bath of molten metal. This type of process is usually carried out in a side blown converter such as a Peirce-Smith converter wherein oxygen-enriched air is injected through 20 to 50 single pipe tuyeres, having a diameter between 25 to 50 mm located along the axis of the reactor more or less in a horizontal position. Usually, oxygen is added to the pure air to a maximum enrichment level of less than 30% oxygen in air. Above this level of enrichment, the refractory wear at the tuyere line becomes excessive.
In 1988, a survey of world copper smelters indicated that of 41 smelters using 134 converters, 15 of them used oxygen enrichment of the blast either occasionally or on a regular basis. The maximum reported enrichment level was 28% oxygen.
It is well known by the man skilled in the art that when air comprising about 30% oxygen or more, is injected in a copper converter, the refractory wear at the tuyere tip line becomes excessive. This deterioration has been explained in an article entitled "Accretion Growth at the Tuyeres of a Peirce-Smith Copper Converter" by A. A. Bustos et al., the Centre of Metallurgical Process Engineering, the University of British Columbia, as published in Canadian Metallurgical Quarterly, Vol. 27, No. 1, pp. 7-21, 1988, Canadian Institute of Mining and Metallurgy, Pergamon Press plc.
In this article is disclosed a mathematical model predicting the size and shape of accretions produced during converting. The results from this model which agree well with industrial experience, indicate that 36% oxygen enrichment level is the maximum theoretical limit compatible with the formation of a protective layer of solid material around the periphery of the tuyere refractory, as explained in said article. Thereafter, the tuyere line is unprotected, which is detrimental to refractory life.
However, in order to avoid the destruction of the refractory lining, as explained hereabove, the industry using copper enrichment converters has not processed an enrichment of more than 28% of oxygen through the air injected through the converter.
U.S. Pat. No. 3,990,899 discloses a converting process for the extraction of heavy non-ferrous metals from their ores, such as copper, nickel, cobalt or lead sulfides.
The conversion is carried out by injecting oxygen of industrial purity (which means oxygen produced cryogenically from air, i.e., having a purity of about 99.9% vol. O.sub.2 --at least) through the bath of molten metal to make the conversion from ore to pure metal with the formation of, e.g., some slag. The refractory lining in the vicinity of the tuyere output is protected by injecting a shielding hydrocarbon or sulfur dioxide or water. To protect the injectors and refractory lining, a shielding fluid such as sulfur dioxide is injected in the molten bath until the sulfur content of the molten bath is about less than 5%, at which time the sulfur dioxide is replaced with hydrocarbon gas. The injection of both SO.sub.2 and hydrocarbon gas, which might generate CO at high temperature, because those two gases are reactive with oxygen, is an environmental issue today.
U.S. Pat. No. 3,990,890 relates to refining molten copper matte with an enriched oxygen blow. This process comprises two phases: during a first phase of the refining, the oxygen content of the oxidizing gas at a pressure in the range of 6 to 20 bar is varied as necessary up to pure oxygen to maintain the temperature of the matte in the range of 1280.degree. C. to 1420.degree. C. and still promote sufficient stirring of the molten bath by the injected oxidizing gas highly enriched with oxygen for a second phase of the refining while still maintaining the temperature in the aforementioned range by adjusting the oxygen content of the oxidizing gas. By injecting a cooling agent, which is a carbon containing material, while blowing the oxidizing gas at much higher pressure than those used before, the oxygen content of the oxidizing gas can be increased far beyond the limits previously imposed in copper matte conversion processes. This high pressure oxygen containing gas expands in the tuyere, due to its high pressure, which creates a cooling effect of the tuyere, but this cooling effect might sometimes be too strong and reinforce the clogging effect (mushroom creation) that this process is supposed to reduce.
U.S. Pat. No. 4,023,781 discloses a tuyere made of three concentric pipes for refining metal in molten stat by injection of oxygen in the central tuyere while protective medium are injected in the outside tuyeres. These tuyeres are individually movable longitudinally into the vessel in order to avoid their premature replacement.
U.S. Pat. No. 4,657,586 discloses copper refining by submerged injection of oxygen and fuel in a concentric tuyere wherein between at least 100% and not more than 150% of the oxygen required for complete conversion of the injected fuel is injected (which means between 66% to 100% of fuel compared to the stoichiometric quantity of oxygen needed for complete combustion of said fuel). Most of the oxygen is thus for the purpose of combusting the fuel to create heat in the molten bath which comprises oxygen containing impurities. It is also disclosed a process wherein the amount of oxygen might be less than that required for complete combustion with the fuel which means that the process disclosed in this patent does not need oxygen injection for the purpose of refining copper.
U.S. Pat. No. 3,844,768 relates to refining of alloy steels, not to copper or nickel refining. In the steel making industry, it is known, as disclosed in said patent, to use argon (in the so-called AOD process) for decarburization of pig iron, including oxygen and argon mixtures or argon in a shrouding tuyere comprising an inner tuyere wherein oxygen is injected through the molten metal. This type of double tuyere with argon-oxygen injection is disclosed in said patent as being inefficient and there is a strong suggestion of using a double or triple feed tuyere with a peripheral flow of carbon make-up liquid.